I get the sense that someone did a bit of research and found that a 10% mark up is about the sweet spot at which many consumers won't notice that the price they are actually paying is actually more than the price they think they are paying (especially in a grocery store, where most consumers make a large order). With this in mind, I don't see how this practice can be anything but deceptive. The goal is clearly to get consumers to agree to buy an item on the condition of its advertised price and then squeeze a few extra bucks out of them by jacking up the price at the checkout counter in the hopes that they don't realize. Or if they do notice the price difference, many consumers may very well decide to buy the item anyhow given that they are already at the checkout counter and it would be more effort than it's worth to put the item back and go buy it elsewhere. If the mark up were any higher, I imagine that more consumers would notice and fewer would be willing to go through with the transaction.